The invention relates to an optical sensor for detecting wetting of a surface.
Optical sensors of this generic type are known. They serve for instance to control a motor vehicle light system and/or windshield wiper system. Measurement value detection is done essentially by an optoeletronic principle. Light from the visible range or the infrared range is coupled into the windshield from the inside of the windshield. The unmoistened outer surface reflects the light, which reaches a receiver. To increase the efficiency, the light is shone in in such a way that total reflection takes place on the outside. The total reflection is disturbed by the wetting of the outer surface with water. It is a common feature of all the known versions that the input and output of the electromagnetic waves take place at spatially markedly separate points, and that the sensor element and the evaluation electronics are accommodated in a common housing.
Error-free signal detection can then be accomplished only if the optical sensor is mounted in a region of the windshield that is cleaned by the windshield wiper system. Therefore in some vehicle types, the sensor has to be mounted at a distance of up to 15 cm from the upper edge of the windshield. A disadvantage of this is that the sensor housing in these cases is within the field of view of the driver and is perceived as annoying because of the lack of transparency. Miniaturization is not possible, since for timely detection of wetting, for instance when it is beginning to rain, a sensor region approximately 4-5 cm2 in area is necessary.
The optical sensor of the invention having the characteristics of the main claim has the advantage in particular that the sensor region need not be disposed immediately where the evaluation electronics are. Because the light is carried between the at least one transmitter and the at least one receiver bidirectionally to the sensor region via a light-carrying element, and a retroreflector is disposed in the sensor region, the sensor region can be disposed at a distance from the transmitter or receiver. The reflection of the sensor signals occurs at the retroreflector, which is preferably shaped from a transparent material. The sensor components that for technical reasons cannot be made from a transparent material, in particular the transmitter and the receiver, can now, in a preferred use as a rain sensor, be disposed outside the field of view of the driver. Thus the visual appearance of the sensor, particularly if it is used as a rain sensor in motor vehicles, can be designed to be less noticeable. Furthermore, a common input and output point of the light is obtained.
Advantageous embodiments for the light-carrying element are monomode or multimode optical waveguides of glass or plastic, either singly or in bundled form. Also plates or suitably shaped bodies of glass or plastic, shaped so that they can carry light. To optimize the light carrying, it is advantageous to apply a coating to the light-carrying element that has a coefficient of refraction that is less by at least a few percent than the light-carrying element. As a result, the total reflection required for carrying light does not take place at the surface of the light-carrying element but rather at the boundary face between the coating and the core material of the light-carrying element. The delivery and return of the light can also be done via one common optical waveguide or separate optical waveguides that are disposed side by side or one above the other.
For inputting the beam of light from the light-carrying element into the windshield and vice versa, a coupling element is provided, which can preferably also be embodied in one piece with the light-carrying element. The beam of light is deflected in such a way that it meets the boundary face of the surface with at least the limit angle of the total reflection. Advantageous embodiments can be elbows, prisms, or a roughened underside of the light-carrying element.
The reflection of the beam of light is advantageously effected via prismatic reflectors. They can be preferably disposed as microstructures with dimensions between 2 xcexcm and 100 xcexcm in circular segments or in strips. Instead of a prismatic reflector, a reflective dye or glass beads embedded in plastic can also be used. If transparency is not necessary, then the prisms can preferably be replaced by mirrored surfaces, and in particular by concave mirror segments, which focus the beam of light onto the output point.
The reflection can also be achieved a hologram that is applied to the window or is for instance glued in the form of a film into the windshield. This provides mechanical protection of the hologram, and the windshield need not have something glued to its inside surface. Furthermore, the hologram film on the surface of the windshield cannot cause any light reflections, and thus it is not so visible to the driver and is thus less annoying.
A further option is for the adhesive film disposed in the windshield to be embodied itself as a hologram film in a suitable region. It is also especially advantageous that the present sensor principle can be employed in slightly modified form to prove the presence of various kinds of measurement substances, in the form of liquid, aerosol, in solution, or in gaseous form. Then a substance is applied in the sensor region that in the presence of the measurement substance reacts by changing its coefficient of refraction or color. The resultant refraction, absorption or reflection of the beam of light in the sensor region causes a signal change, which can be recorded in the receiver. In this embodiment, retroreflectors and the substrate of the sensitive substance can preferably be embodied as a structural unit.
It is also advantageous that because of the spatial separation of the electronics and the retroreflector, a harmful influence of the measurement substance on the components of the electronics can be averted, since the contact with the measurement substance takes place only via the sensor region. The material comprising the substrate can preferably be glass or a transparent plastic. What is essential is that this body have a surface at which total reflection occurs.
The construction according to the invention makes it possible for there to be markedly fewer components in the region of the primary measurement value detection. As a result, a greater range of variation of the sensor shape and size is also attained.
Further preferred features will become apparent from the other characteristics recited in the dependent claims.